VERTEBRATES. 4 19 



through the intermediate gradations, presents no striking contrast to 

 that of the larger teeth. But regarded independently, the extreme 

 examples bear strongest contrast to the teeth first noticed above ; yet, 

 in view of the evidence actually within reach, there seems to be no 

 question as to their specific identity. The latter teeth are about .20 inch 

 in lateral diameter, and .30 in hight. 



A tooth of large size measures in lateral diameter 1.15 inch, antero- 

 posterior diameter of crown .70, elevatiou of crown .40. 



The description of this remarkable form is based upon a fine suite of 

 specimens, mainly belonging to the collections of Mr. Springer and 

 Mr. Wachsmuth, all of which were derived from the same locality. 

 Varying greatly in size and contour, we have seldom met with similar 

 instances where specific identity is so unequivocally demonstrable as in 

 the present case. The large teeth present features in outline and gen- 

 eral contour, which suggest relationship with some of the peculiar 

 Pseplwdi of the same horizon ; but to what extent this resemblance 

 should influence the determination of their generic relations, we have 

 not been able to decide They, however, apparently hold the same rela- 

 tion to Pseplwdus as Helodus planus of the European Carboniferous 

 limestone does. But such interpretation of their affinities necessarily 

 presumes a combination of Gochliodont and Gestraciont features of the 

 most extraordinary latitude, such, indeed, as obtains in no other family, 

 ancient or modern, of the class to which these remains belong. 



Messrs. Wachsmuth and Springer have placed in our hands a large 

 collection of the above form, and from the study of this material we 

 have encountered many difficulties in the way of determining the limits 

 of the individual specimens in their relation to other and perhaps gene- 

 rically allied forms. Worn examples, and which, unfortunately, is the 

 prevalent condition of these teeth, are scarcely distinguishable from 

 other forms allied to Helodus biformis, K and W., regarding the affini- 

 ties of which latter with Pseplwdus of the same deposits, we have 

 scarcely a doubt. In a communication with which we have been 

 favored from Lord Enniskillen, it appears that there is no longer rea- 

 son to doubt the specific identity of the Irish Carboniferous teeth origi- 

 nally described under the names Helodus planus and Pseplwdus magnus, 

 Agass. We have ourselves observed similar combinations in some of 

 the American representatives of Pseplwdus, as for example, that Irjin 

 the Chester limestone, to which Messrs. Newberry and Worthen 

 gave the name Aspidodus crenulatus. Considering the apparent affini- 

 ties of the teeth under consideration, however marked the contrast of 

 individuals, we have been led, provisionally, to refer them to the same 

 genus. 



Position and locality : Kinderhook formation ; the same form, appa- 

 rently, occurring in both fish- bed horizons; Burlington, Iowa. 



